Silver halide photographic elements contain light sensitive silver halide in a hydrophilic emulsion. An image is formed in the element by exposing the silver halide to light, or to other actinic radiation, and developing the exposed silver halide to reduce it to elemental silver.
In color photographic elements a dye image is formed as a consequence of silver halide development by one of several different processes. The most common is to allow a by-product of silver halide development, oxidized silver halide developing agent, to react with a dye forming compound called a coupler. The silver and unreacted silver halide are then removed from the photographic element, leaving a dye image.
In either case, formation of the image commonly involves liquid processing with aqueous solutions that must penetrate the surface of the element to come into contact with silver halide and coupler. Thus, gelatin, and similar natural or synthetic hydrophilic polymers, have proven to be the binders of choice for silver halide photographic elements. Unfortunately, when gelatin, and similar polymers, are formulated so as to facilitate contact between the silver halide crystal and aqueous processing solutions, they are not as tough and mar-resistant as would be desired for something that is handled in the way that an imaged photographic element may be handled. Thus, the imaged element can be easily marked by fingerprints, it can be scratched or torn and it can swell or otherwise deform when it is contacted with liquids.
There have been attempts over the years to provide protective layers for gelatin based photographic systems that will protect the images from damage by water or aqueous solutions. U.S. Pat. No. 2,173,480 describes a method of applying a colloidal suspension to moist film as the last step of photographic processing before drying. A series of patents describes methods of solvent coating a protective layer on the image after photographic processing is completed and are described in U.S. Pat. Nos. 2,259,009, 2,331,746, 2,798,004, 3,113,867, 3,190,197, 3,415,670 and 3,733,293. The application of UV-polymerizable monomers and oligomers on processed image followed by radiation exposure to form crosslinked protective layer is described U.S. Pat. Nos. 4,092,173, 4,171,979, 4,333,998 and 4,426,431. Major drawbacks for the solvent coating method and the radiation cure method are the health and environmental concern of those chemicals to the coating operator and the instability and relatively short shelf life of the coating solutions. U.S. Pat. Nos. 3,397,980, 3,697,277 and 4,999,266 describe methods of laminating polymeric sheet film on the processed image as the protective layer. U.S. Pat. No. 5,447,832 describes the use of a protective layer containing mixture of high and low Tg latices as the water-resistance layer to preserve the antistat property of the V.sub.2 O.sub.5 layer through photographic processing. This protective layer is not applicable to the image formation layers since it will detrimentally inhibit the photographic processing. U.S. Pat. No. 2,706,686 describes a lacquer finish for photographic emulsions, with the aim of providing water- and fingerprint-resistance by coating the emulsion, prior to exposure, with a porous layer that has a high degree of water permeability to the processing solutions. After processing, the lacquer layer is fused and coalesced into a continuous, impervious coating. The porous layer is achieved by coating a mixture of a lacquer and a solid removable extender (ammonium carbonate), and removing the extender by sublimation or, dissolution during processing. The overcoat as described is coated as a suspension in an organic solvent, and thus is not compatible with current manufacturing of photographic products. U.S. Pat. No. 3,443,946 provides a roughened (matte) scratch-protective layer, but not a water-impermeable one. U.S. Pat. No. 3,502,501 provides protection against mechanical damage only; the layer in question contains a majority of hydrophilic polymeric materials, and must be permeable to water in order to maintain processability. U.S. Pat. No. 5,179,147 likewise provides a layer that is not water-protective.
In U.S. Pat. No. 5,804,341 a protective overcoat is formed by applying a coating, in the presence of a electric field, charged, clear polymeric particles to an imaged silver halide element so as to cause the particles to adhere to a surface of the imaged element and then fusing the polymeric particles. In U.S. Pat. No. 5,856,051 a protective overcoat is formed by applying a coating of hydrophobic polymer particles having an average size of 0.01 to 1 microns, a melting temperature of from 55 to 200.degree. C. at a weight percent of 30 to 95, and gelatin at a weight percent of 5 to 70 over a silver halide light-sensitive emulsion layer. The silver halide light sensitive emulsion layer is developed to provide an imaged photographic element. The hydrophobic polymer particles are then fused to form a protective overcoat. However, there remains a need to provide protective overcoats on photographic elements without a fusing step.
Thus, there is a need to control the coefficient of friction of the overcoat layer in order to be run through photofinishing equipment and/or handled by the end-users properly. In addition, the desire to enhance the resistance of the protective layer to scratches and to prevent prints from adhering to on another in high temperature and/or high humidity is still greatly recognized.
The temperature and residence time of photographic coating in the drying section of photofinishing equipment in the trade vary from 50.degree. C. to 70.degree. C. and from 30 seconds to 2.5 minutes. The actual temperature of gelatin coating during drying is much lower than the temperature set for the dryer due to the evaporation of water. In addition, it is necessary to be free of volatile organic compound (VOC) in the formulation to be user and environmental friendly. Under these stringent requirements, it appears that an aqueous colloidal dispersion of a water insoluble polymeric material is an appropriate system for this technology. Water soluble materials will not provide any water resistance property.
U.S. Pat. No. 2,719,791 describes the use of an aqueous dispersion of organic plastic material, which yields a water and vapor permeable coating on drying. However, it is known that when dispersions of low Tg material (Tg&lt;25.degree. C.) are used to obtain a water resistance protective coating, the surface of the protective coating has an undesirable tacky characteristic, which generally degrades other physical properties, such as print blocking, fingerprinting, dust attraction and high scratch propensity. When dispersions of high Tg materials (Tg&gt;25.degree. C.) are used, it is not possible to form a continuous water resistance layer on the prints under the drying condition described above. U.S. Pat. No. 2,751,315 also describes the use of an aqueous dispersion of copolymer materials. It was recognized in this patent that low Tg materials were not suitable and therefore higher Tg polymers in combination with an organic solvent were used in order to form a water-resistant protective coating. The organic solvent that is released from the formulation during drying creates an environmental concern if used in the current photofinishing laboratories. U.S. Pat. No. 2,956,877 describes a method of applying a solution to a photographic image that solublizes the processing reagents from the photographic products as well as forming a protective coating on its surface. The acid groups on the polymer degrades the water resistant property of the final protective layer, and the organic solvent required in the formulation is not suitable for high volume photofinishing laboratories.
There remains a need for an aqueous coatable, water-resistant protective coating that can be easily coated on imaged processed photographic products and dried into a continuous coating under drying conditions typical of photographic processing equipment while not releasing volatile organic compounds and while also providing a low coefficient of friction, improved resistance to scratching and high-temperature blocking, without any adverse effect on the gloss of the photographic product.